Drumboy, try vibrating the mechanism as you work the key by tapping on the safe with a rubber mallet or a block of wood.
The key does not really turn very much to release the lock so try the lever as well as you are tapping(two person job).
Once you get it open you should remove the door cover and get inside and see what's causing the issue. Could be loose parts or metal debris,lack of lubricant etc but it should be fairly easy to resolve